Downhole completion system sealing against the cap layer

1. A downhole completion system for completing a well through a top formation having an impermeable top cap layer having an upper end and a lower end, the impermeable top cap layer being positioned above a hydrocarbon-containing reservoir, and

a borehole extending through the cap layer to provide an inner cap layer face,

the system comprising a first well tubular metal structure configured to be arranged in the borehole, the first well tubular metal structure comprising:

a first annular barrier and a second annular barrier, each annular barrier comprising:

a tubular metal part, the tubular metal part being mounted as part of the first well tubular metal structure,

an expandable tubular surrounding the tubular metal part, each end section of the expandable tubular being connected with the tubular metal part,

an annular barrier space between the tubular metal part and the expandable tubular, and

an expansion opening in the tubular metal part through which pressurised fluid is configured to pass for expanding the expandable tubular and bringing the annular barrier from an unexpanded position to an expanded position,

wherein the first annular barrier is configured and arranged to be positioned at the upper end of the cap layer, and in the expanded position, the expandable tubular of the first annular barrier overlaps the cap layer, and the second annular barrier is configured and arranged to be positioned at the lower end of the cap layer, and in the expanded position, the expandable tubular of the second annular barrier overlaps the cap layer to create a confined space between the first and second annular barriers.

2. The downhole completion system according to claim 1 , wherein the downhole completion system is a cementless downhole completion system.

3. The completion system according to claim 1 , wherein the confined space is cementless.

4. The downhole completion system according to claim 1 , wherein the first well tubular metal structure comprises a sensor unit configured to identify the impermeable cap layer.

5. The downhole completion system according to claim 1 , wherein in the expanded position, the first annular barrier, the second annular barrier, the first well tubular metal structure and the cap layer are configured to enclose the confined space.

6. The downhole completion system according to claim 1 , wherein the first well tubular metal structure comprises a sensor unit arranged between the first annular barrier and the second annular barrier and being configured to measure a property of a fluid in the confined space.

7. The downhole completion system according to claim 6 , wherein the sensor unit is comprised in the first annular barrier or the second annular barrier.

8. The downhole completion system according to claim 6 , wherein the sensor unit comprises a communication device configured to communicate sensor data.

9. The downhole completion system according to claim 6 , wherein the sensor unit comprises a pressure sensor or a temperature sensor.

10. The downhole completion system according to claim 1 , further comprising a pressurisation device for pressurising the first well tubular metal structure.

11. The downhole completion system according to claim 1 , further comprising one or more third annular barrier(s) arranged between the first annular barrier and the second annular barrier.

12. The downhole completion system according to claim 1 , wherein the first annular barrier or the second annular barrier comprises a valve device in fluid communication with the expansion opening.

13. The downhole completion system according to claim 1 , further comprising a second well tubular metal structure extending at least partly within the first well tubular metal structure and adapted to extend below the cap layer.

14. The downhole completion system according to claim 1 , wherein one of the first or second annular barriers is made solely from a metal material.

15. The downhole completion system according to claim 1 , further comprising a second well tubular metal structure being suspended from the first well tubular metal structure.

16. The downhole completion system according to claim 15 , wherein the second well tubular metal structure is a liner hanger.

17. A completion method for a downhole completion system comprising:

identifying an impermeable top cap layer positioned above a hydrocarbon-containing reservoir,

introducing the first well tubular metal structure into the borehole,

arranging a first annular barrier and a second annular barrier at least partly opposite the impermeable cap layer so that an expandable tubular of each of the first annular barrier and the second annular barrier overlaps the impermeable cap layer, and

expanding the expandable tubular of the first annular barrier and the second annular barrier to abut the impermeable cap layer to enclose a confined space between the first and second annular barriers.

18. The completion method according to claim 17 , further comprising pressurising the confined space to a predetermined pressure.

19. The completion method according to claim 17 , further comprising determining if pressure in the confined space is kept substantially constant over a period of time to verify the sealing properties of at least one of the annular barriers against the cap layer.

20. The completion method according to claim 17 , further comprising shifting a valve device of one of the first or second annular barriers from a first position providing fluid communication from an inside of the first well tubular metal structure to an annular barrier space to a second position providing fluid communication between the annular barrier space and the confined space.

21. The completion method according to claim 17 , wherein the confined space is cementless.